Messages in this thread | | | Date | Tue, 7 Jun 2022 14:54:14 +0200 | Subject | Re: [PATCH AUTOSEL 5.16 07/28] x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0 | From | Paolo Bonzini <> |
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On 6/6/22 23:27, Peter Xu wrote: > On Mon, Jun 06, 2022 at 06:18:12PM +0200, Paolo Bonzini wrote: >>> However there seems to be something missing at least to me, on why it'll >>> fail a migration from 5.15 (without this patch) to 5.18 (with this patch). >>> In my test case, user_xfeatures will be 0x7 (FP|SSE|YMM) if without this >>> patch, but 0x0 if with it. >> >> What CPU model are you using for the VM? > > I didn't specify it, assuming it's qemu64 with no extra parameters.
Ok, so indeed it lacks AVX and this patch can have an effect.
>> For example, if the source lacks this patch but the destination has it, >> the source will transmit YMM registers, but the destination will fail to >> set them if they are not available for the selected CPU model. >> >> See the commit message: "As a bonus, it will also fail if userspace tries to >> set fpu features (with the KVM_SET_XSAVE ioctl) that are not compatible to >> the guest configuration. Such features will never be returned by >> KVM_GET_XSAVE or KVM_GET_XSAVE2." > > IIUC you meant we should have failed KVM_SET_XSAVE when they're not aligned > (probably by failing validate_user_xstate_header when checking against the > user_xfeatures on dest host). But that's probably not my case, because here > KVM_SET_XSAVE succeeded, it's just that the guest gets a double fault after > the precopy migration completes (or for postcopy when the switchover is > done).
Difficult to say what's happening without seeing at least the guest code around the double fault (above you said "fail a migration" and I thought that was a different scenario than the double fault), and possibly which was the first exception that contributed to the double fault.
Paolo
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